I'm not a stuff person, but the book I would be most upset if I lost and had to replace would be Ptolus, which, in two versions and across three RPG systems/editions, has powered my gaming since 2006.
I'm pretty sure I posted here back then that I could see this book being the last campaign book I ever really needed* and it looks like I was right.
* "Need" doesn't preclude buying, enjoying and using Freeport and Radiant Citadel in that time, of course, and I'm awaiting shipment of Obojima this winter. But I've never stopped using Ptolus and doubt I ever will.